It is often desirable to determine the direction of a radiation source, e.g., gamma radiation, fast neutrons, and so on such as in tomography, astronomy, and civil defense applications. However, because most forms of radiation interact with matter through the processes of the photoelectric effect, Compton scattering and pair production, it has been difficult to accurately detect and locate the radiation source.
One approach is the collimated instrument with shielding that restricts angular acceptance of radiation and reduces background contributions from other directions, so that maximum output is obtained only when an aperture in the shielding is aligned with a radiation source. The disadvantages of collimated instruments include distortion of incoming radiation by interactions with the collimator walls, a small solid angle of acceptance when a high directional resolution is necessary thus reducing radiation intensity, and poor angular resolution when a large solid angle of acceptance is necessary.
Other approaches include the filament-type detector that obtains directional information by aligning filament axes toward the radiation source to provide a directional output. This approach is described in Chupp et al., "A Direction Neutron Detector for Space Research Use," IEEE Transactions on Nuclear Science NS-13, pp. 468-477, February 1966. The filament array approach uses forward-peaked angular distribution of protons from n-p collisions to obtain directional effects, as described in Stetson et al., "A Directional Scintillation Counter for Neutrons," 6 Nuclear Instruments and Methods, pp. 94-95, 1960.
Often, weight and portability are important considerations in selecting a sensor. For example, space applications require lightweight devices, and simplicity is desired since repair is not feasible. These same considerations are also applicable to mobile detectors, particularly hand-held devices or those requiring access to restricted locations.
Another approach is the fast neutron directional detector described in Byrd et. al, U.S. Pat. No. 5,345,084, entitled, "Directional Fast-Neutron Detector," issued Sep. 6, 1994, in which several omnidirectional fast neutron radiation detectors are closely packed to form a segmented symmetric detector and a processor arithmetically combines the incident radiation counts from the detectors to output a signal functionally related to a direction of a source for radiation. Output radiation counts are combined by subtracting counts from the detectors having front-back symmetry and subtracting counts from the detectors having left right symmetry. Using this approach, the resulting differences form a vector quantity indicating radiation source direction, but this detector is limited to detecting the direction of fast neutrons and has a low directional resolution.
The long-felt need for a directional detector or sensors of radiation sources with a high angle of acceptance and high resolution for detecting different types or intensities of radiation sources was met with Kronenberg et. al, U.S. Pat. No. 5,665,970 entitled, "Directional Radiation Detector and Imager." Although those devices provide much improved accuracy, a scan-time of an hour or more for independent measurements was still required after positioning the device at several varying angles. This limitation of measurement time could inhibit practical, real-time field use, or other intense or dangerous circumstances.
The present invention addresses and overcomes these sensitivity and time of measurement drawbacks, shortcomings and limitations with a rotatable scintillator assembly which provides a faster and more sensitive radiation sensor device than any of the prior art devices and techniques. This invention's scintillation-type radiation sensor provides a much-improved sensitivity below background levels, rather than merely at background levels, within a few minutes rather than the hours previously required. This much-improved, rapid sensitivity is made possible by a rotatable scintillation unit, interacting with the measured radiation to produce light flashes, together with a stationary photomultiplier tube that converts the light flashes to electrical pulses and instrumentation for counting pulses within the photomultiplier unit. In one embodiment of the present invention, the instrumentation is connected to a processor to afford even greater portability for field use.
In addition to the above-cited Kronenberg et al., U.S. Pat. No. 5,665,970 "Directional Radiation Detector and Imager," issued to the present inventors on Sep. 9, 1997, which is incorporated herein by reference, the following publications also illustrate prior art devices:
S. Kronenberg, et al. "Directional Detector For Arrays Of Gamma Ray and k-ray Sources," Nuclear Instruments and Methods in Physics Research, Section A, 378, pp. 531-540, 1996;
S. Kronenberg et al. "Locating and Imaging Sources of Gamma and X-radiation Directly or Through Thick Shields," Nuclear Instruments and Methods in Physics Research, Section A, 387, pp. 401-409, (1997); and
S. Kronenberg et al. "High Angular Resolution Sensing Of Gamma Rays In Space," International Society for Photo-Optical Instrumentation Engineers Journal, 3116, pp. 49-56, July 1997.